In order to understand the role of ADCC-Abs in protection from influenza illness and to determine whether influenza vaccines elicit ADCC-Abs, we quantified ADCC-Abs in serum samples from adults who received a dose of monovalent 2009 H1N1 pandemic (H1N1pdm09) inactivated subunit vaccine (ISV), or live-attenuated influenza vaccine (LAIV) or had laboratory-confirmed H1N1pdm09 infection. We also measured ADCC-Abs in children who either received a dose of trivalent seasonal ISV followed by trivalent seasonal LAIV or two doses of LAIV. Finally, we assessed the ability of low and high ADCC-Abs titers to protect adults from experimental challenge with influenza A/Wisconsin/67/131/2005 (H3N2) virus. We found that adults and children that received a dose of ISV had a robust increase in ADCC-Ab titers to both recombinant (r)HA protein and homologous virus-infected cells. There was no detectable increase in ADCC-Abs to rHA or virus-infected cells in adults and children that received LAIV. Higher titers of pre-existing ADCC-Abs were associated with lower virus replication and a significant reduction in total symptom scores in experimentally infected adults. Thus, ADCC-Abs increased following experimental influenza virus infection in adults and ISV administration in both children and adults. Live attenuated influenza vaccine (LAIV) and inactivated influenza vaccine (IIV) are available for children. An earlier paper by our group demonstrated decreased shedding in children immunized with seasonal LAIV, as compared to IIV, following challenge with LAIV one month after the initial vaccination. In children challenged with LAIV after IIV, 10 of 15 shed one or more of the influenza virus strains in the trivalent vaccine with 21 of 45 possible strains recovered. In contrast, when LAIV recipients were challenged with LAIV, one of 11 shed virus although that child shed all three strains after both vaccinations. The efficacy of LAIV and IIV is poorly explained by either single or composite immune responses to vaccination. Protective biomarkers were therefore studied in response to LAIV or IIV followed by LAIV challenge in children. Serum and mucosal responses to LAIV or IIV were analyzed using immunologic assays to assess both quantitative and functional responses. Cytokines and chemokines were measured in nasal washes collected before vaccination, on days 2, 4, and 7 after initial LAIV, and again after LAIV challenge using a 63-multiplex Luminex panel. We found that patterns of immunity induced by LAIV and IIV differed significantly. Serum responses induced by IIV, including hemagglutination inhibition, did not correlate with detection or quantitation of LAIV on subsequent challenge. Modalities that induced sterilizing immunity seen after LAIV challenge could not be defined by any measurements of mucosal or serum antibodies induced by the initial LAIV immunization. No single cytokine or chemokine was predictive of protection. From this thorough analysis, we concluded that the mechanism of protective immunity observed after LAIV could not be defined and traditional measurements of immunity to IIV did not correlate with protection against an LAIV challenge. A live attenuated influenza virus (LAIV) vaccine is licensed for healthy adults 2-49 years of age. This vaccine is administered by nasal spray. Neutralizing antibody in the serum has been found to be a correlate of protection for TIV, but the immune correlates of protection for LAIV are not known. Defining the origin and nature of transcriptional responses to LAIV in upper respiratory tract will be a highly informative first step in a systems approach toward understanding the molecular basis of viral replication restriction and the regulation of the local mucosal immune responses following LAIV administration. In FY13, in collaboration with colleagues at Stanford University, we undertook a natural history study using a systems biology approach to identify LAIV replication niches among a variety of URT cell types and characterize the host immune response to LAIV. Data analysis is in progress.